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Phosphotungstic Acid Modified Expanded PTFE Based Nafion Composites

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Phosphotungstic Acid Modified Expanded PTFE Based Nafion Composites Journal of New Materials for Electrochemical Systems 14, 217-222 (2011) © J. New Mat. Electrochem. Systems Phosphotungstic Acid Modified Expanded PTFE based Nafion Composites K. Pattabiraman and K. Ramya Centre For Fuel Cell Technology, ARC- International, IIT M Research Park, Phase I, 2nd Floor, No.6 Kanagam Road, Taramani, Chennai 600113, India. Received: May 06, 2011, Accepted: May 30, 2011, Available online: July 06, 2011 Abstract: Composite membranes have been prepared by impregnation of PFSI( perfluorosulphonic acid ionomer) solution (Nafion®) into the expanded polytetrafluoroethylene (EPTFE) matrix. The matrix was modified by Na/Naphthalene treatment and with phosphotung- stic acid (PTA). The matrix modifications helped in reducing the contact angle thereby improving the impregnation process by reducing the hydrophobicity of the matrix. The fuel cell made with these composite membranes showed that the performance of PTA modified matrix is comparable to that of Na/ naphthalene modified matrix. Further the performances of matrix modified membranes were higher than that of the as received matrix-composites. PTA treatment offers a simple method of matrix modification to achieve high performance. Keywords: Polymer electrolyte membrane fuel cells, Composite ionomers, expanded PTFE, Phosphotungstic acid, surface modification 1. INTRODUCTION high temperature performance of the most commonly used per- fluorosulphonic acid based polymer electrolyte in fuel cells Perfluorosulphonic acid ionomers (PFSI) are fast ion conductors [13,14]. Malhotra and Dutta [15] showed that incorporation of and are widely used as an electrolyte in many electrochemical inorganic solid acids (phosphotungstic acid) in Nafion resulted in devices [1]. Recent literature reports have shown that composite improved water retention capacity due to additional acid sites and membranes of PFSI offer many advantages in terms of lower cost, good fuel cell performance at elevated temperatures. Kannan et al higher thermal stability and good mechanical strength [2], im- [16] have prepared modified Nafion membranes with phospho- proved durability and performance [3], reduced swelling character- silicate for improved water retention and proton conduction. They istics [4] etc. A composite membrane prepared using a porous also found that samples with higher phosphate content exhibited framework substrate with the pores filled with a PFSI such as high water uptake and proton conductivity. Nafion offers all the benefits described above [5,6]. Nafion has a Literature studies thus cite the advantages of using hydrophobic tetrafluoroethylene backbone and hydrophilic side Nafion/EPTFE composite membranes and the use of heteropolya- chains that aggregate making it difficult to fill the porous EPTFE cids as additives for improvement in properties. Further surface substrates with Nafion solutions. Various methods to improve modified EPTFE have been shown to have better wetting charac- impregnation include use of solvents having solubility parameters teristics and improved properties due to better interactions between suitable to backbone/ ionomer[2,7], use of surfactants [8], surface the substrate and Nafion. As EPTFE substrate is very stable it modification of EPTFE through etching [9,10] etc. requires a strong radical ion (sodium Naphthalene radical) to ab- Heteropolyacids (HPAs) are the most attractive inorganic modi- stract the fluoride ion from backbone. The process requires the use fiers because these materials in crystalline form are highly conduc- of a number of solvents and the localized heat generated during tive [11] and thermally stable. The use of heteropolyacids in defluorination may damage the membrane. A simpler process for preparation of composite polymer electrolytes for fuel cell offer surface modification would hence be advantages. This paper sug- many advantages as they improve conductivity and decrease the gests a non-reactive surface modification method using phospho- dependence of the membrane on humidity for optimum perform- tungstic acid. The EPTFE substrate treated with PTA solution ance [12]. In addition, they have also been proven to improve the would have a coating of PTA molecules in the pores of the sample. *To whom correspondence should be addressed: Email: [email protected] Due to good interactions between the PTA and Nafion all the Phone: +91-44-66632710 , Fax: +91-44-66632702 pores of substrate would get coated and an improved membrane 217 218 K. Pattabiraman and K. Ramya / J. New Mat. Electrochem. Systems would be obtained. The results of these modifications are presented in this paper. To the best of our knowledge this is the only paper where PTA has been used first to treat the matrix to be coated with Nafion later. 2. EXPERIMENTAL 2.1. Preparation of Nafion/EPTFE composites Porous EPTFE ( Yeu Ming Tai Chemical Industrial Co., Ltd., Taiwan, thickness 25±5µm, porosity 85%) was first washed with alcohol and dried in oven. The EPTFE film was then mounted on a frame and soaked in acetone for 30 min. Nafion solution (5%) ob- tained from Dupont Co, with an equivalent weight of 1000 was used for impregnating the porous EPTFE film. To minimize the aggregation of Nafion in the solution it was mixed with a high boil- ing solvent ( dimethylformamide / etylene glycol) to make the solu- tion into 2.5% of polymer in the solvent. The Nafion loading in the porous EPTFE film was maintained constant at 2mg/cm2 for all the membranes. Impregnation was carried out in steps to ensure that all the pores of the membrane were fully blocked to avoid gas cross- over. Membrane post treatment procedure included pressing the mem- branes at 30kg/cm2 at 120°C for 3 minutes followed by annealing. The membranes were soaked in water for 24 hrs and then in dilute H2SO4 for 4 hrs. The membranes were washed free of acid and used for further studies. The average thickness of the membranes was 27±5µm. For preparation of etched membranes the procedure adopted by Tang et al [9] and Jie et al [10] were used. The etched membranes were impregnated with Nafion using the procedure mentioned above. Fig 1 shows the schematic for the preparation of PTA modi- fied membranes. The impregnation procedure and the post treat- ment procedures followed were the same as that for other mem- Figure 1. Schematic for the preparation of PTA modified PTFE branes. composite 2.2. Characterization of Nafion/EPTFE composites The morphology of the surface of the composite membranes was studied using scanning electron microscope (SEM). The surface of fabrication of the MEA. the sample was coated with gold before the morphology of the The composite membranes prepared by the above techniques membranes was observed. EDAX analysis was carried out to study were used for the preparation of membrane electrode assemblies. the various elements present on the surface. FTIR studies were The membrane electrode assemblies were made by a proprietary carried out to study the functional groups present and the interac- technique developed in our lab using 40% Pt/C( Arora Mathey, tion between the various groups present. Thermogravimetry studies India), 5% Nafion Solution (DuPont, USA) and Carbon substrate were carried out to study the thermal stability of the various mem- from Ballard, USA. The catalyst layers were coated on a Gas diffu- branes prepared. Contact angle measurements were carried out to sion layer by brush and the catalyst loading (total) was 1mg/sq cm. 2 study the effect of various surface modifications on the porous The performance data were obtained from a 6.75 cm active area PTFE substrate. Conductivity measurements were carried out in the cell. Pure hydrogen and air were used as fuel and the oxidant. The transverse direction using two probe techniques. The membrane cells were operated at ambient pressure conditions. The inlet gases resistance was obtained by AC impedance technique in the fre- were humidified to temperatures higher by 10°C and 5°C than cell quency range of 100kHz to 100 Hz. temperature for hydrogen and air respectively. 2.3. Gas permeability and Fuel cell polarization 3. RESULTS AND DISCUSSION studies Three types of composite membranes based on Nafion/EPTFE Gas permeability across the membranes were tested by assem- were prepared. This included impregnation of pure EPTFE with bling the membrane in a fuel cell set up and passing the gas on one Nafion solution, fabrication of etched EPTFE/ Nafion composite 2 side of the cell. The test consisted of applying a 1kg/cm differen- and third non-interacting PTA modified EPTFE/Nafion composite. tial over pressure on one side while capping the other inlet and Although etching is a good surface modification method, it requires leading the outlet to a water reservoir[3]. If any bubble was ob- the use of Na/naphthalene radical synthesis and many solvents. The served in the water reservoir the membranes were not used for process may also not lead to uniform etching when large area mem- Phosphotungstic Acid Modified Expanded PTFE based Nafion Composites / J. New Mat. Electrochem. Systems 219 a) b) c) d) e) Figure 2. SEM pictures of a) Plain EPTFE, b) Etched EPTFE and c) PTA modified EPTFE d) Nafion coated EPTFE e) Transverse section of Nafion- ePTFE branes have to be processed. A non-reactive surface modification son to that of pure PTFE film the pores have become smaller. The method is suggested and the properties are compared with that decrease in the pore size has been reported to be due to structure obtained in other methods. The PTA modification was expected to and stress change as a result of Na/Naphthalene treatment [9]. The improve coatability and reduce interfacial resistance due to de- reduction in pore size was also reported to be advantageous for crease in contact angle as a result of PTA coating on the pores and impregnation of the pores due to higher capillary force. The smooth surface of the matrix when treated with a dilute solution of PTA in surface of the etched film shows that the film has not been dam- a wetting liquid and due to interaction of Nafion with PTA.
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